The semiconductor processing industry makes use of diffusion furnaces to deposit various coatings on the surfaces of wafers. The coatings are produced from gases introduced into the reaction vessels of the diffusion furnaces. The gases pyrolytically decompose, are surface catalyzed by the wafers, and deposit as thin films. For any of the known CVD and other processes, the quality, uniformity, yield, and repeatability of the deposited thin films critically depend on the temperature, quantity, purity, flow rate, flow pattern and distribution of the gases.
Some heretofore known diffusion furnaces have one or more vertically stacked generally cylindrical and horizontally extending furnace tubes as shown, for example, in commonly assigned issued United States utility patent application Ser. No. 836,294, incorporated herein by reference. Wafers are loaded in "boats", usually fabricated of quartz or other refractory material, designed to hold a plurality of wafers. Multiple boats are placed on boat-loading mechanisms operative to automatically insert and remove the boat loaded wafers into and out the furnace tubes. Reactant gases are usually introduced at one end of the furnace tubes and are introduced through longitudinally extending gas injection tubes as shown, for example, in commonly assigned U.S. Pat. No. 4,573,431, incorporated herein by reference. The gases flow longitudinally past multiple downstream wafers before being exhausted to a vacuum port coupled to the other end of the process tubes. The downstream wafers of each furnace load of wafers are thereby exposed to excess unreacted reactants, reaction by-products, gas particulate impurities and other contaminants, and different gas quantities and flow rates.
Other heretofore known semiconductor wafer processing furnaces have substantially a two-part, removable cover and base defining a Bell-jar like reaction vessel as disclosed, for example, in U.S. Pat. Nos. 4,545,327 and 4,547,404, each incorporated herein by reference. Heating elements are carefully positioned along inside walls surrounding and spaced from the covers of the Bell-jar like reaction vessels. In the interspace between the inside walls and covers, a positive pressure is established to prevent injectant gases from flowing backwards from the Bell-jar like reaction vessels towards, and contaminating, the heating coils and the inside walls of the semiconductor wafer processing furnaces.
An upstanding pedestal and a spaced apart linearly-extending vacuum port are provided inside the Bell-jar like reaction vessels. The pedestals include a gas showerhead spaced from and confronting the vacuum ports inside the reaction vessels. Gases are released from the showerheads and are attracted by the vacuum ports. The gases flow into the vacuum ports past and parallel to the surfaces of wafers placed on the base. The gas flow patterns that exist between the showerheads and the confronting vacuum ports are, however, such that different quantities of gases and flow rates deposit as different thickness thin films in dependence on the particular location of the wafers producing undesirable coating non-uniformities wafer to wafer and batch to batch.
Another disadvantage is that the processing throughput of the Bell-jar like reaction vessels is restricted. Access to the reaction vessels involves lifting the covers off of the bases, and reseating them, after having manually lowered or lifted the boats onto and off of the bases. The lifting, boat placement and removal, and cover seating operations are both time and labor intensive and to that extent limit the processing throughput.
In addition to a slower than desirable processing throughput, the Bell-jar like reaction vessels have a quite limited capacity. For each production run, only a few boats of wafers are capable of being processed within their limited interior space. To achieve a high yield, production runs must be either repeatedly made or more machines must be kept in continuous operation. Both avenues require that considerable acquisition, production, and maintenance costs be incurred.
Particulates are a problem for both the longitudinal and the Bell-jar like diffusion furnaces. The sizes of structures being fabricated are being decreased so as to get more and more devices onto less wafer area to improve circuit density and circuit performance. The sizes of some particulates are commensurate with the sizes of some of the high-speed microstructures being fabricated. As the probability of damage increases with the number of devices on a chip, once innocuous particles can spoil whole batches of high circuit density high performance wafers. In the heretofore known horizontally extending semiconductor processing furnaces, the gases flow from upstream wafers past downstream wafers to the vacuum port. The downstream wafers are thereby exposed to entrained particulates, and therewith to possible degradation. In the heretofore known Bell-jar like reaction vessels, the upper gas showerheads and confronting vacuum ports therebelow expose the wafers on the base to a potentially damaging rain of particulates as the gases, and the entrained particles and other contaminates, flow past the wafers.